1. Field of the Invention
The present invention relates to a method of judging a deterioration of a catalytic converter installed in the exhaust system of an internal combustion engine.
2. Description of the Prior Art
Internal combustion engines such as automotive engines are required to incorporate a catalytic converter in their exhaust system for purifying an exhaust gas as much as possible from the standpoint of environmental protection. There has been proposed a system combined with an internal combustion engine and having an air-fuel ratio sensor disposed upstream of a catalytic converter for detecting the air-fuel ratio of an exhaust gas and an exhaust gas sensor disposed downstream of the catalytic converter for detecting the content of a certain component in the exhaust gas, e.g., the concentration of oxygen contained in the exhaust gas. The proposed system controls the air-fuel ratio of the internal combustion engine to enable the catalytic converter to perform an optimum emission purifying capability according to a feedback control process based on detected output signals from the air-fuel ratio sensor and the exhaust gas sensor. For details of the proposed system, reference should be made to Japanese patent application No. 5-321721 corresponding to U.S. Pat. No. 5,426,935 and Japanese patent application No. 8-84047 corresponding to U.S. patent application Ser. No. 08/833091.
The system operates on the basis of a finding that the catalytic converter can purify the exhaust gas optimally by controlling the air-fuel ratio of the exhaust gas that enters the catalytic converter such that the concentration of oxygen contained in the exhaust gas that has passed through the catalytic converter will be of a certain constant value. Based on such a finding, the system determines a target air-fuel ratio for the exhaust gas that enters the catalytic converter such that the detected output signal from the exhaust gas sensor (oxygen concentration sensor) positioned downstream of the catalytic converter will be of a certain constant value. When the detected output signal from the exhaust gas sensor (oxygen concentration sensor) positioned downstream of the catalytic converter is of the certain constant value, the target air-fuel ratio may be corrected such that the detected output signal from the air-fuel ratio sensor positioned upstream of the catalytic converter will be of a stoicheometric air-fuel ratio. The air-fuel ratio of an air-fuel mixture to be burned in the internal combustion engine is controlled by a feedback control process, i.e., the rate of fuel supplied to the internal combustion engine is controlled by a feedback control process, for converging the detected output signal from the air-fuel ratio sensor toward the target air-fuel ratio.
The emission purifying capability of the catalytic converter decreases because of aging, use of inappropriate fuels, etc. When the catalytic converter is deteriorated to some extent, it is desirable to determine the deterioration.
Conventional processes of judging a deterioration of catalytic converters are disclosed in Japanese patent No. 2,526,640 corresponding to U.S. Pat. No. 5,088,281 and Japanese laid-open patent publication No. 7-19033.
According to the process shown in Japanese patent No. 2,526,640 which corresponds to U.S. Pat. No. 5,088,281, while the internal combustion engine is operating usually, since detected output signals from the oxygen concentration sensors disposed respectively upstream and down-stream of the catalytic converter are reversed between leaner and richer values of the air-fuel ratio, the rate of fuel supplied to the internal combustion engine is increased or reduced depending on a reversal of the detected output signals from the oxygen concentration sensors for thereby controlling. the air-fuel ratio according to a feedback control process so as to be kept in the vicinity of the stoicheometric air-fuel ratio. When the internal combustion engine increases its output power, the rate of fuel supplied to the internal combustion engine is determined by an open-loop control process. In the open-loop control process, a period of time from the time when the detected output signal from the oxygen concentration sensor upstream of the catalytic converter is reversed from the leaner value to the richer value to the time when the detected output signal from the oxygen concentration sensor down-stream of the catalytic converter is reversed from the leaner value to the richer value is measured, and a deterioration of the catalytic converter is determined on the basis of the measured period of time.
The process shown in Japanese laid-open patent publication No. 7-19033 also employs the reversal of the detected output signals from the oxygen concentration sensors upstream and down-stream of the catalytic converter, as with the process disclosed in Japanese patent No. 2,526,640. According to this process, the air-fuel ratio of the internal combustion engine is controlled by a feedback control process so as to be maintained at a target value depending on the detected output signals from the oxygen concentration sensors. If certain conditions are satisfied, i.e., if the load and rotational speed of the internal combustion engine fall in predetermined ranges, while in the feedback control process, then a deterioration of the catalytic converter is determined on the basis of the detected output signals from the oxygen concentration sensors upstream and down-stream of the catalytic converter. Specifically, with the air-fuel ratio being corrected at a greater degree in the feedback control process than usually, the time of a response delay of the reversal of the detected output signal from the down-stream oxygen concentration sensor with respect to the reversal of the detected output signal from the upstream oxygen concentration sensor, and the period of the reversal of the detected output signal from the down-stream oxygen concentration sensor are measured, and a deterioration of the catalytic converter is determined on the basis of the measurements.
However, these processes of judging a deterioration of the catalytic converter have been disadvantageous as described below.
According to the processes disclosed in Japanese patent No. 2,526,640 corresponding to U.S. Pat. No. 5,088,281 and Japanese laid-open patent publication No. 7-19033, while the air-fuel ratio of the internal combustion engine is being controlled by the feedback control process based on the detected output signals from the oxygen concentration sensors upstream and down-stream of the catalytic converter to enable the catalytic converter to perform an optimum emission purifying capability, it is impossible to judge a deterioration of the catalytic converter. In order to judge a deterioration of the catalytic converter, it is necessary to control the air-fuel ratio of the internal combustion engine according to the open-loop control process or correct the air-fuel ratio at a greater degree in the feed-back control process than usually. Stated otherwise, judging a deterioration of the catalytic converter requires the air-fuel ratio of the internal combustion engine to be controlled in a special mode different from a usual mode. As a consequence, the control system for the internal combustion engine is relatively complex.
Particularly, according to the processes disclosed in Japanese patent No. 2,526,640 corresponding to U.S. Pat. No. 5,088,281 and Japanese laid-open patent publication No. 7-19033, inasmuch as the air-fuel ratio of the internal combustion engine needs to be controlled in a special (dedicated) mode, which is different from the usual mode for controlling the air-fuel ratio to enable the catalytic converter to perform an optimum emission purifying capability, in order to judge a deterioration of the catalytic converter, it is difficult for the catalytic converter to keep the optimum emission purifying capability when a deterioration of the catalytic converter is to be judged.
Furthermore, because it is impossible to judge a deterioration of the catalytic converter while the internal combustion engine is operating usually with the air-fuel ratio being controlled by the feedback control process to enable catalytic converter to keep the optimum emission purifying capability, there are few opportunities to judge a deterioration of the catalytic converter, and hence the determination of a deterioration of the catalytic converter is not reliable.